In many fields of optical investigation of specimens, such as forensic analysis or in materials science, useful information can often be obtained from viewing a specimen between a pair of crossed polarizing filters (or “crossed polars” as they are known in the art) under a microscope, in order to observe any interference colours (also known as retardation colours) produced by the specimen if it is birefringent. As those skilled in the art will appreciate, when viewed between crossed polars a birefringent specimen often appears bright against a dark background, and typically exhibits a multicoloured pattern characteristic of the underlying composition and structure of the specimen. Many polymer materials are birefringent, because the polymer molecules are ‘frozen’ in a more-or-less parallel configuration when the plastic is extruded or moulded. Many crystalline materials are also birefringent, such as calcite, for example. Some fibres (e.g. as used in fabrics) are also birefringent. For example, cotton fibres are birefringent because of high levels of cellulosic material in the fibre's secondary cell wall. By observing a birefringent specimen between crossed polars and analysing the colours and patterns seen, the material can be characterised. For example, the pattern often indicates the manner in which a polymer material was extruded or moulded. Furthermore, if the thickness of the specimen is known, it is often possible to establish its birefringence from the interference colours that it exhibits between crossed polars. Thus, observation of these colours can be a particularly useful technique in forensic investigation (e.g. enabling fibre samples to be compared with one another, or linked to a clothing manufacturer or supplier), although it also has extensive applications in materials science and other fields of scientific and technological research.
By way of some background to forensic investigation techniques, it is relatively easy for a criminal to avoid leaving his or her fingermarks at a crime scene. Similarly, he or she can often guard against leaving detectable quantities of DNA behind when committing a crime. In contrast, it is virtually impossible for a criminal to be present at a crime scene without the two-way transfer of fibres occurring between the scene and the person concerned. This has meant that fibres evidence has been pivotal in solving a number of serious crimes, e.g. involving kidnapping, murder or rape. However, despite its advantages, in the main, fibres evidence is not routinely used in volume crime cases. This is for a number of reasons, one of which is the difficulty of examining fibres quickly, easily and cost-effectively, without risking contamination (which could cause the evidence to be rejected in court). The difficulty of examining fibres has also hampered the compilation of a suitable fibres database for forensic investigation purposes.
There are a number of means by which fibres evidence can be collected from crime scenes. These include brushing, scraping and vacuuming. However, the most common method in use is tape-lifting—a process that involves using an adhesive polymer tape (similar to Sellotape®) to remove fibres from the surface to be sampled. This tape is then adhered to a suitable backing sheet so as to capture the fibres between the tape and the sheet, packaged, and then sent to a laboratory for analysis of the fibres.
The fibres can be analysed by a range of techniques, including infra-red and Raman spectroscopy, pyrolysis-gas chromatography, and polarised-light microscopy. Of these, the technique that is most commonly used is polarised-light microscopy to observe interference colours and establish birefringence values, as it is highly discriminating and non-destructive. However, the conventional polymer tape that is used to collect the fibres evidence interferes with this technique, as the tape itself is birefringent too. As a consequence, to establish the fibres' birefringence, the fibres are individually dissected from the tape, removed, and then placed into a suitable mounting medium between a microscope slide and a cover slip. As a result of all these steps, and the need to use skilled personnel to conduct the examination with care such that the evidence will be properly admissible in court, the process is both time-consuming and expensive. Moreover, throughout the dissection and remounting process, there is a risk that the fibres could be lost or contaminated, which would render them useless as evidence in court, and could potentially affect the outcome of a case.
Accordingly there is a need for a quicker, easier and reliable process for capturing birefringent fibres or other such materials and examining the interference colours that they exhibit when illuminated between crossed polars. There is also a desire for a tape or film that does not meaningfully alter such colours produced by a birefringent specimen such as a fibre captured thereon.
Background art is provided in WO 03/010569 A2, WO 2006/038023 A1, U.S. Pat. No. 5,677,024 and U.S. Pat. No. 4,544,583. WO 03/010569 A2 discloses an optical filter. WO 2006/038023 A1 discloses a sample-lifting tape and a method for the collection of a sample from an area, for example for forensic examination. U.S. Pat. No. 5,677,024 discloses a laminate having particular polarisation characteristics, and U.S. Pat. No. 4,544,583 discloses a birefringence-free arrangement of plastic foils.